For all telecommunication systems, there is a variety of reconfiguration procedures present. These procedures can be divided into two main groups, based on the nature of the parameters to be reconfigured, namely soft and physical reconfigurations. Physical reconfigurations deal with reconfigurations of a physical nature, such as radio bearer reconfiguration, transport channel reconfiguration, physical channel reconfiguration. Soft reconfigurations deal with non-physical reconfigurations, such as for example security parameter reconfiguration. For a typical scenario of 3GPP specifications, these two types of reconfigurations are treated somewhat different and consequently suffer from different and separate problems.
The present disclosure will focus on soft reconfigurations, in particular security reconfigurations in relation to 3 GPP specifications TS 25.331 V8.7.0 section 8.1.12.4b, “Cell Update Procedure During Security Reconfiguration”. One area of improvement concerns the case of dropped calls due to a mismatch of security configurations between the network and a user terminal such as a mobile phone, as a consequence of cell reselection procedure during the security reconfiguration.
For connected 3G users in the so called CELL_FACH state or mode trying to set up a multi-RAB speech call, call drop occurs if a Cell-Update cell reselection procedure coincides with the Security Mode procedure. To further clarify, the CELL_FACH state or mode is one of the radio resource control connected modes or states of operation. As such, for a user equipment in the CELL_FACH state the following is applicable.                No dedicated physical channel is allocated to the UE.        The UE continuously monitors a FACH in the downlink.        The UE is assigned a default common or shared transport channel in the uplink (e.g. RACH) that it can use anytime according to the access procedure for that transport channel.        The position of the UE is known by UTRAN on cell level according to the cell where the UE last made a cell update.        
It should be noted that Security Mode procedure includes negotiating which ciphering and integrity protection scheme the concerned parties e.g. user equipment and network node are to use for communication. A mismatch or misalignment of security configuration between two parties e.g. user terminal and a network, will ultimately lead to a dropped call since the parties are unable to communicate with each other.
During UE mobility, two such scenarios are possible:                1) Security Reconfiguration during Cell Update procedures, i.e. Security Mode Command is received in a user equipment (UE) from a network just after a Cell Update message is sent from the UE to the network.        2) Cell Update procedure during Security Reconfiguration, i.e. a Cell Update message is sent from the UE while the Security Mode procedure is still ongoing.        
Prior art, as represented by 3GPP specification TS 25.331 V8.7.0, section 8.1.12.4b, “Cell update procedure during security reconfiguration,” section 8.1.12.2.2, “Integrity protection configuration change,” and section 8.3.1.9b, “Security reconfiguration during Cell update procedure,” describe how a user equipment UE or mobile and network should handle these two cases; however, there is room for improvement to further reduce the risk of calls being dropped as a result of 3GPP specification limitations.
In general, all above mentioned problems are related to misalignment or mismatch in security (ciphering/integrity) settings when an ongoing Security Mode procedure is aborted, primarily due to Cell Update cell-reselection. If both UE and radio network controller (RNC) abort the security reconfiguration or if neither aborts, a network solution could easily handle this case. However, due to different race conditions occurring between cell update and security mode procedures, UE may abort reconfiguration but not the RNC, and vice versa. The result is an Integrity Protection (and/or ciphering) misalignment resulting in call drop.
With reference to FIG. 1, known Security Mode procedures from the UE point of view will be described. Within the time span designated “A”, it is clear from the 3GPP specification TS 25.331 V8.7.0 section 8.1.12.4b, “Cell Update Procedure During Security Reconfiguration” that the UE shall abort the ongoing security mode procedure if a Cell Update needs to be sent. This Cell Update can be triggered by any of the following scenarios:
a) re-selection to a new cell
b) re-entering service area
c) periodical cell-update
d) to inform the network of a UE failure (“physical channel failure” or “RLC unrecoverable error”)
For the present disclosure, the case of a user equipment aborting an ongoing security reconfiguration procedure due to reselection to a new cell will be exploited.
When it comes to the time span designated “B” above, 3GPP specifications are somewhat unclear and also limited regarding UE security configuration behavior. If a CellUpdate message is sent during the security procedure, after securityModeComplete, but before the L2ACK received, then as above, the UE shall abort the ongoing Security Mode procedure (3GPP specification TS 25.331 V8.7.0 section 8.1.12.4b, “Cell Update Procedure During Security Reconfiguration”) with special handling for integrity parameter COUNT-I. Some other vague guidance is given by a statement targeting the RNC [2] (3GPP specification TS 25.331 V8.7.0 section 8.1.12.2.2, “Integrity protection configuration change”), in which it is stated that the network (NW) should be aware that the UE “may” abort the security procedure.
Aborting the security procedure in the UE at this point however is not favorable, since the UE has just acknowledged to the RNC (in Security Mode Complete message) that the security reconfiguration is already performed even though the security reconfiguration is not yet fully applied in the UE until the L2ACK for securityModeComplete is received from the RNC (i.e. it is a grey area limitation in the prior art as represented by 3GPP specification TS 25.331 V8.7.0 section 8.1.12.4b, “Cell Update Procedure During Security Reconfiguration”).
If the UE aborts the security reconfiguration after RNC has received the Security Mode Complete, the new security reconfiguration will be applied by the RNC. Hence there is a security mismatch, leading to call drop (as evidenced from live network analysis). The dropped call is due to the fact that the UE and the network at this point in time are using different security configurations and are unable to communicate.